Impact of El Niño: How dry can 2014be?

What is El Niño? El Niño is a band of anomalously warm ocean water temperatures that periodically develop off the Pacific coast of South America. El niño is Spanish for “the boy,” and the capitalised term El Niño refers to the Christ Child, Jesus, because periodic warming in the Pacific near South America is usually detected around Christmas. The 'El Niño–Southern Oscillation' (ENSO) refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean (El Niño and La Niña) and in air surface pressure in the tropical western Pacific. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific. The mechanisms that cause the oscillation are still being studied. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific and Indian Ocean, are the most affected.  
From a historical perspective, weak, moderate, and strong El Niño and La Niña years are listed as follows.

El Niño Years:
Weak: 52-53, 53-54, 58-59, 69-70, 76-77, 77-78, 04-05, 06-07
Moderate: 51-52, 63-64, 68-69, 86-87, 87-88, 91-92, 94-95, 02-03, 09-10
Strong: 57-58, 65-66, 72-73, 82-83, 97-98

La Niña Years:
Weak: 50-51, 54-55, 56-57, 64-65, 67-68, 71-72, 74-75, 83-84, 84-85, 95-96, 00-01, 05-06, 08-09, 11-12
Moderate: 55-56, 70-71, 98-99, 07-08
Strong: 73-74, 75-76, 88-89, 99-00, 10-11

How do El Niño and La Niña impact Bangladesh climate? Bangladesh normally faces a deficit of rainfall (and drought conditions) during any major (strong) El Niño event. Many previous major El Niño years, such as 1951, 1958, 1972, 1983, 1993, 1997, and, most recently, 2009, recorded significant deficits in the monsoon seasonal-average (Jul-Aug-Sep) rainfall. For example (based on a previous study conducted by the writer in 2003), 1951, 1958, and 1972 recorded about 38-, 48-, and 10-percentage less rainfall. The major (strong) La Niña years --1964, 1973, 1988 and 1998 -- recorded excessive rainfall. The increases were as follows: 4% in 1964; 8% in 1973; 30% in 1988; and 10% in 1998. In particular, Bangladesh is always found to be wetter than normal during any La Niña year (weak, moderate, or strong); however, Bangladesh is not always drier than normal during any El Niño year. For example, the year 1987 was a moderate El Niño year and the country experienced flooding rather than drought.
What is the causal connection between El Niño/La Niña and Bangladesh climate? During an El Niño year, the trade wind weakens and as a result the whole Ganges-Brahmaputra-Meghna (GBM) basin experiences less rainfall. The weakening of trade winds depends on how strong, moderate, or weak the event is. The deficiency of rainfall causes Bangladesh rivers to dry out because of low-flow and results in severe drought. On the other hand, during any La Niña year, the trade wind strengthens and as a result there is a significant increase in rainfall along the greater GBM basins, which cause flooding along the whole catchments. Again, the strengthening of trade winds depends on how strong (e.g., weak, moderate, or strong) the event is. This, in turn, severely floods Bangladesh, as it is the lowest riparian country in these basins.
How is El Niño developing now (2014)? Right now, it looks like an El Niño is developing. According to the Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI), the likelihood of El Niño developing is 50% or more during the summer or fall of 2014 (see Figure).
Forecasters at NOAA's Climate Prediction Center and the International Research Institute for Climate and Society released this ENSO forecast on March 6. (Courtesy CPC/IRI.)
Most of the world's major seasonal prediction models now push the Pacific into an El Niño temperature regime by July. Some scientists even argue that there is 67% chance of developing an El Niño by July–September of 2014, while others argue that it may develop by October–December of 2014. Researchers are closely watching the Pacific, including the interplay among the various factors, and making their own assessments of how likely El Niño might be.
What can happen in 2014? If a weak El Niño event develops then this will cause basin-wide lower than the average rainfall during the months of June to September, which may result in slightly drier than normal weather in Bangladesh. Similar to the weak event, a moderate event also can cause drier than normal weather for the same time period. However, as mentioned before, there are some exceptions in the case of moderate event (e.g., 1987). Note that the 1987 event is a rare event, and there were probably other open factors that caused flooding in that moderate El Niño year. Further research is needed here. However, if a strong El Niño event develops then a drought, similar to that of 1982-83 and 1997-98, is most likely to occur. In that case the GBM basin-wide rainfall will be exceptionally low, rivers will dry out, and the drought will continue for a long time. Based on some recent news report, it seems likely that the country is already drier than normal. Further, the river water sharing conflicts between Bangladesh and the neighbouring countries may also aggravate the problem.
What is the verdict now? While most of the El Niño events over the last 15 years have been on the weaker side, currently some conditions in the western tropical Pacific are now strikingly similar to those that preceded the two strongest El Niño events of the last century: 1982–83 and 1997–98. There is uncertainty in any forecast but these trends are still of concern for all of us. If the strong El Niño doesn't take shape in the next several months, however, we may not see it this year at all. If it does happen, then it could have a huge global impact, which will also seriously affect Bangladesh. Therefore, like many other International Research and Applications Climate Centers, we at the 'Pacific ENSO Applications Climate Center' are constantly monitoring this event and regularly updating early warnings for vulnerable communities in the Pacific Islands. I believe local climate experts and concerned applications agencies in Bangladesh are also equally vigilant in monitoring this on-going El Niño event.
Finally, it is important to keep in mind that in probabilistic seasonal climate predictions “we never have perfect predictability”; however, the goal is to find ways to take advantage of the information we do have.  

The writer Principal Research Scientist (Graduate Faculty), 'Pacific ENSO Applications Climate University of Hawaii Manoa, USA.  E-mail: [email protected]